Human activity is largely to blame for the worsening water shortages in the western United States over the past half-century,
a new study shows. The analysis of climate trends that influence the availability of freshwater shows that humans are responsible
for 60% of the observed changes.

According to climate and river data stretching back to 1950, the western states have experienced several changes that make
water — already one of the region's most valuable natural commodities — more scarce.

Snowpack in mountain ranges such as the Rockies is diminishing, average minimum temperatures are rising, and spring run-off
in major rivers such as the Colorado river is happening earlier. All of these changes look set to make dry summers even harsher
for residents of thirsty cities such as Las Vegas in Nevada, and Los Angeles and San Diego in California.

Research led by Tim Barnett of the Scripps Institution of Oceanography in La Jolla, California, now shows that these changes
cannot be explained without taking human activities — which have raised greenhouse-gas levels — into account. Barnett is so
worried by the situation that he says development in these regions should be capped. "We're using all the water there is right
now, and people still want to build bigger cities," he says.

Unnatural causes

Barnett and his team used computer models to simulate the natural behaviour of the water cycle in the region. They compared
this to the trends seen over the past 50 years, to see whether they can be attributed to natural causes. The answer, they
report in this week's Science, is no1.

Barnett and his colleagues then factored in the global warming that has occurred as a result of burning fossil fuels over
the past half-century, using figures calculated by the Intergovernmental Panel on Climate Change (IPCC). This time, the models
matched more closely the weather trends experienced in the western US states.

Overall, says Barnett, human greenhouse emissions explain around 60% of the trends in freshwater behaviour. He puts the odds
of natural causes being responsible at "somewhere between 1 in 100 and 1 in 1,000".

Already arid

"What they've got is quite scary," comments Ian Cluckie, a hydrologist at the University of Bristol, UK. The trend looks
likely to make water shortages even more severe for residents of southern California, Arizona and New Mexico, he says. These
naturally arid regions already receive a lot of their water by 'north–south transfer' — careful manipulation of water flow
through damming and aquifer management — from wetter regions to the north, Cluckie says. "This area only survives by major
water transfer and it's going to get much, much worse," he adds.

Just how much worse is difficult to say. Much of the rainfall in the region occurs through thunderstorms and these are too
localized to be predicted accurately by climate models, which are usually organized in grids with squares several hundred
kilometres across.

Other regions likely to suffer similar problems include the entire area surrounding the Mediterranean Sea, and the southern
parts of Africa, says Chris Milly, of the US Geological Survey. Syria, for example, looks set to suffer a more than 40% drop
in freshwater runoff by 2050, according to computer models based on IPCC emissions predictions (see ).

Time to act

The analysis of Barnett and his colleagues doesn't take into account changes that could be made through careful water management,
Cluckie says. Although there is already a lot of strain on the water supply, there may be some room for prioritization, by
juggling the amounts of water used for hydroelectric power versus domestic use, for example.

Other options available for avoiding drought include treatment of poor-quality water such as household 'greywater', increasing
the price of water supplies and desalination in coastal areas. But perhaps the most important will be water conservation.

That will require political leadership, which is not yet present in all quarters, says Barnett. "In California they are pretty
clued-in and have been leading the way, but they have been stymied by Congress," he says.